In technical heating methods, medium wave infrared radiation is often used in the most various fields in order to feed process heat. This being medium wave sources of radiation such as spiral-wound filaments which are enclosed by a clear glass protecting tube translucent for infrared radiation or several such tubes often serve as heat sources. These tubes are open on the front side since it is not necessary to protect the spiral-wound filaments from the ambient air—they can be operated in air in the medium wave infrared range without oxidation and remain stable for several years. The glass protecting tube is rather primarily used for the electrical protection.
Since the ambient air is substantially without influence on the spiral-wound filament, a break of the glass protecting tube does not result, differently from for example a break of the glass bulb of a traditional filament bulb, compulsorily to a change of the electrical behaviour of the infrared radiator heat source. Thus the risk does exist that such a break remains unnoticed.
Apart from the fact that with a broken glass protecting tube the electric insulation is not given any longer, it is necessary when using these infrared radiator heat sources in sensitive processes to avoid a soiling of the material to be radiated with broken glass pieces or to stop immediately the process in case of a glass break so that all the parts which can come into contact with glass broken pieces can be eliminated and so that a soiling of further process material does not take place.
In U.S. Pat. No. 3,533,704 a method and a device are disclosed for which glass objects, for example glass tubes, and are tested with respect to breaks by means of light in a kind of quality control after the manufacturing. To this purpose light is introduced frontally into the glass object and the light coming out of the glass is checked with a detector. Intensity fluctuations indicate a break.
In U.S. Pat. No. 3,777,171 a similar method and a comparable device are indicated which serve for the detection of occlusions or non-homogeneities in glass objects, for example in glass tubes.
The methods and device mentioned in the two mentioned printed documents serve for the quality control and are not provided for the use for the control of a glass protecting tube during the operation of an infrared radiator heat source or another insitu control.
Before this background the aim of the invention is to create a method and a device with which the integrity of a glass protecting tube can be reliably checked for a spiral-wound filament of an infrared radiator heat source during operation and a break thereof can be surely ascertained.